Multi-strand ropes are used in many different applications for offshore and onshore projects, and must work safely throughout their life. As these ropes are often used for lifting operations, they can be subject to extensive bending and wear. Ropes used in offshore operations, where lifting equipment can be operating at depths of 2-3 km, are especially subject to wear and fatigue issues due to active heave compensation (“AHC”) systems that are typically used for these offshore operations.
When ropes start to wear, individual strands making up the rope can break, thereby decreasing the cross-sectional area and strength of the rope and increasing the chances of a catastrophic incident or failure of the rope. Often these broken strands are inner strands, and therefore cannot be seen by a simple visual inspection. To inspect the entire rope (including the inside), a device which measures the cross-sectional area of the rope can be connected around the rope. The entire rope is then typically run through the device, and the cross-sectional area measurements are used to make an assessment of whether the rope needs replacement. Such a device is described in EP0845672 A1, which uses a magnetic testing device for obtaining a damage index of a rope by passing the rope through the device. The device includes sensors to detect local faults (e.g., nicked or broken wires) and sensors to detect the loss of metallic area, for example, from wear or corrosion. A real-time indication of a damage level of the rope is produced on a hand-held readout unit from processing the data from local fault sensors and loss of metallic area sensors (as well as a length transducer to reference the local faults) and display the damage index. Other similar devices for detecting a defect in a rope can be found in WO201/098756 A1, WO2010/092619 A1, and WO 2004/022469. Each of these disclose devices for detecting a defect in an elevator cable by running the cable through the detection device to detect a weakness or failing in the cable and the relevant location. Each of these devices takes a measurement of the existing state of the rope or cable at only a moment in time to make a determination regarding defects or damage from this measurement. None sense and/or store usage data over normal operations.
If the rope or cable does not need immediate replacement, the area measurements are often used to recommend that the rope be replaced after a specific time period. The time period is typically a conservative estimate based on classification society guidelines to guarantee the safety of the systems and the state of the rope at the time of inspection.